Golf balls are generally classified as either solid or wound. Solid golf balls are typically made with a solid core encased by a cover, both of which can have multiple layers, such as a dual core having a solid center and an outer core layer, or a multi-layer cover having an inner and outer cover layer. Additionally, one or more intermediate layers may be disposed between the core and cover. The golf ball cores and/or centers are constructed with a thermoset rubber, typically a polybutadiene-based composition. Wound golf balls, precursors to the solid golf ball, typically include a solid, hollow or fluid-filled center, surrounded by tensioned elastomeric material and a cover.
While early solid golf balls tended to be more durable and resilient than wound construction, thereby providing better distance, they lacked the softer feel, lower compression and higher spin rate of wound golf balls preferred by accomplished golfers who are better able to control the ball on the green.
More recently, cores of solid golf balls have been heated and crosslinked to create certain characteristics, such as higher or lower compression, which can impact the spin rate of the ball and/or provide better “feel.” These and other characteristics can be tailored to the needs of golfers of different abilities, making the solid golf ball an attractive option. In this regard, from the perspective of a golf ball manufacturer, it is desirable to have cores exhibiting a wide range of properties, such as resilience, durability, spin, and “feel,” because this enables the manufacturer to make and sell many different types of golf balls suited to differing levels of ability.
Heretofore, most single core golf ball cores have had a conventional hard-to-soft hardness gradient from the surface of the core to the center of the core. The patent literature contains a number of references that discuss a hard surface to soft center hardness gradient across a golf ball core.
U.S. Pat. No. 4,650,193 to Molitor et al. generally discloses a hardness gradient in the surface layers of a core by surface treating a slug of curable elastomer with a cure-altering agent and subsequently molding the slug into a core. This treatment allegedly creates a core with two zones of different compositions, the first part being the hard, resilient, central portion of the core, which was left untreated, and the second being the soft, deformable, outer layer of the core, which was treated by the cure-altering agent. The two “layers” or regions of the core are integral with one another and, as a result, achieve the effect of a gradient of soft surface to hard center.
U.S. Pat. No. 3,784,209 to Berman, et al. generally discloses a soft-to-hard hardness gradient. The '209 patent discloses a non-homogenous, molded golf ball with a core of “mixed” elastomers. A center sphere of uncured elastomeric material is surrounded by a compatible but different uncured elastomer. When both layers of elastomer are concurrently exposed to a curing agent, they become integral with one another, thereby forming a mixed core. The center of this core, having a higher concentration of the first elastomeric material, is harder than the outer layer. One drawback to this method of manufacture is the time-consuming process of creating first elastomer and then a second elastomer and then molding the two together.
Other patents discuss cores that receive a surface treatment to provide a soft ‘skin’. However, since the interior portions of these cores are untreated, they have the similar hard surface to soft center gradient as conventional cores. For example, U.S. Pat. No. 6,113,831 to Nesbitt et al. generally discloses a conventional core and a separate soft skin wrapped around the core. This soft skin is created by exposing the preform slug to steam during the molding process so that a maximum mold temperature exceeds a steam set point, and by controlling exothermic molding temperatures during molding. The skin comprises the radially-outermost 1/32 inch to ¼ inch of the spherical core. U.S. Pat. Nos. 5,976,443 and 5,733,206, both to Nesbitt et al., disclose the addition of water mist to the outside surface of the slug before molding in order to create a soft skin. The water allegedly softens the compression of the core by retarding crosslinking on the core surface, thereby creating an even softer soft skin around the hard central portion.
Additionally, a number of patents disclose multilayer golf ball cores, where each core layer has a different hardness thereby creating a hardness gradient from core layer to core layer.
The cover, while initially designed to protect the golf ball, may also be modified to target a desired spin rate, feel and playability. Conventionally, golf ball covers have been constructed/formed from polymeric materials. For instance, golf ball covers have incorporated balata rubber, either natural, synthetic or a blend of natural and synthetic balata. Other golf balls have incorporated covers which are formed from synthetic polymeric materials such as polyolefins and in particular, polyethylene, polyurethanes, and ionic copolymers of olefins. The latter mentioned ionic copolymers of olefins were commercially introduced in the mid 1960's by E. I. Du Pont de Nemours & Co., Inc., Wilmington, Del. (DuPont) and sold under the trademark “SURLYN.” Golf balls incorporating SURLYN covers are generally described in U.S. Pat. No. 3,454,280. Cover compositions that are based on SURLYN resins are advantageous in that the resulting covers are cut and abrasion resistant compared to the balata covers. While golf balls incorporating SURLYN resin covers are commonly known by players to be more cut resistant than balata covered balls, they traditionally tend to reduce the spin imparted to a golf ball and produce a less desirable “feel” as compared to a balata covered ball. SURLYN resins sold by DuPont typically contain zinc, lithium, magnesium or sodium ions. A number of SURLYN resins, of varying physical properties, are sold by DuPont. The physical properties of these resins are described in technical bulletins that are readily available from DuPont. Mixtures of various SURLYN resins as cover stock materials are likewise highly advantageous. Suitable mixtures for use as cover materials are described in U.S. Pat. No. 3,819,768.
For purposes of control, golfers strike a golf ball in such a manner that the ball has substantial backspin. It is desirable that a golfer be able to impart backspin to a golf ball for purposes of controlling its flight and controlling the action of the ball upon landing on the ground. For example, substantial backspin will make the ball stop once it strikes the landing surface instead of bounding forward. The ability to impart back spin onto a golf ball is related to the deformation of the golf ball cover when struck with a golf club. Generally, the more deformable the cover is, the easier it is to impart spin to the balls. This is particularly true for short or wedge shots.
While this may be accomplished in a hard cover golf ball where the core which is deformable when the cover is struck by a club, it would also be desirable to have a cheaply manufactured golf ball in which cover itself which is temporarily deformable upon impact with the golf club yet meanwhile durable—i.e. scuff resistant.
There are many patents that are directed to golf balls having multiple cover layers in an attempt to provide golf ball covers with versatile playing characteristics. For example, U.S. Pat. No. 4,431,193 relates to a golf ball having a multilayer cover wherein the inner layer is a hard, high flexural modulus ionomer resin and the outer layer is a soft, low flexural modulus ionomer resin, and wherein either or both layers may comprise a foamed ionomer resin. U.S. Pat. No. 5,314,187 also relates to golf balls having multiple layer covers, wherein the outer layer is molded over the inner layer and comprises a blend of balata and an elastomer where the inner layer is an ionomer resin. U.S. Pat. No. 4,919,434 is directed towards a golf ball having a cover which comprises an inner layer and an outer layer each of which is a thermoplastic resin. Preferably the layers comprise thermoplastic resin materials that are capable of fusion bonding with each other. U.S. Pat. No. 5,783,293 discloses a golf ball with a multi-layered cover formed by a co-injection molding process, wherein the golf ball cover comprises an inner and outer layer of a first material and an intermediate layer there between of a second material.
U.S. Pat. Nos. 7,582,025 and 7,427,242 (see Cross Reference to Related Applications herein), disclose golf balls comprising a thermoplastic core wherein the core has been exposed to a gradient-initiating solution comprising a sufficient amount of plasticizer such that the hardness of the core outer surface is less than the hardness of the geometric center to define a negative hardness gradient of 5 Shore C or greater. In the present invention, however, it has been further recognized that desired golf ball characteristics including unique combinations of desired resilience, durability, compression, “feel,” and spin may also be achieved inexpensively in a single layer core or multilayer core golf ball where there is a hardness gradient between a fatty acid and/or fatty acid salt composition treated and/or comprising cover layer surface (inner and/or outer) and at least one of a cover layer different surface, a cover layer region, a core surface and a core region.